Compounds of copper indium diselenide (CIS) with gallium substituted for all or part of the indium (copper indium gallium diselenide or CIGS) are used in photovoltaic devices. For example, CIGS provides absorber layers in thin-film solar cells. CIGS semiconductor materials have a direct band gap that permits strong absorption of solar radiation in the visible range. CIGS cells have demonstrated high efficiencies and good stability as compared to other absorber layer compounds such as cadmium telluride (CdTe) or amorphous silicon (a-Si).
Solar cell devices typically include a substrate, a barrier layer, a back contact layer, a semiconductor layer, alkali materials, an n-type junction buffer layer, an intrinsic transparent oxide layer, and a conducting transparent oxide layer. In a device that utilizes CIGS, the semiconductor layer includes copper, indium, gallium, and selenium. The CIGS layers used for photovoltaic conversion need to have a p-type semiconductor character and good charge transport properties. These charge transport properties are favored by good crystallinity. The CIGS thus need to be at least partially crystallized in order to have sufficient photovoltaic properties for use in the fabrication of solar cells. Crystallized CIGS compounds have a crystallographic structure corresponding to the chalcopyrite or sphalerite systems, generally depending on the deposition temperature.
CIGS thin films can be deposited by various techniques, typically vacuum based. One technique involves the use of precursors. In this technique, intermediate compounds are used and have physicochemical properties that are distinct from those of CIGS and make them incapable of photovoltaic conversion. The precursors are initially deposited in a thin film form, and the thin film is subsequently processed to form the intended CIGS layer. When precursor materials are deposited at a low temperature, the resulting CIGS thin films are weakly crystallized or amorphous. These thin films need to be annealed by supplying heat to improve the crystallization of the CIGS and provide satisfactory charge transport properties. At the temperatures that allow at least partial crystallization of the CIGS, however, one of the constituent elements of the CIGS (selenium) is more volatile than the other elements. It is therefore difficult to obtain crystallized CIGS with the intended composition and stoichiometry without adding selenium during annealing of the precursor layer. Time consuming annealing of the precursor deposits in the presence of selenium excess in the vapor phase is thus needed to form suitable material.
Another technique for depositing CIGS thin films involves vacuum evaporation. Devices formed by this technique often have high photovoltaic conversion efficiencies compared to techniques that use precursor materials. Typically, co-evaporation of the copper, indium, gallium, and selenium is performed in the presence of a substrate. This co-evaporation technique has an advantage in that the content of gallium in the thin film light-absorbing layer can be regulated to achieve the optimum bandgap. Evaporation is a technique that can be difficult to use on the industrial scale, however, particularly because of non-uniformity problems with the thin film deposits over large surface areas and a low efficiency of using the primary materials.
There are additional challenges that arise when using vacuum deposition techniques for depositing CIGS thin films. For example, selenium reacts aggressively with many materials that are typically used in the manufacture of vacuum deposition sources especially at elevated temperatures. Accordingly, the materials and the mechanical design of deposition sources used in a selenium environment are carefully considered.
Additionally, undesirable accumulation of deposition material in the vicinity of the effusion orifice of a vacuum deposition sources can occur under certain deposition conditions. Typically, such deposition conditions include one or more of high deposition temperatures and high deposition rates such as those used for deposition of high temperature metals or semiconductors materials, such as copper, for example. Continued accumulation of deposition material can reduce the area of the effusion orifice and thereby reduce the deposition rate. Ultimately, continued accumulation of deposition material can effectively close the effusion orifice so the deposition rate is unacceptably low or non-existent.